High resolution observations of SiO masers: Comparing the spatial distribution at 43 and 86 GHz

We present sub-milliarcsecond observations of SiO masers in the late-type stars IRC + 10011 and cCyg. We have used the NRAO Very Long Baseline Array (VLBA) to map the 43 GHz ( v = 1,2 J = 1-0) and the 86 GHz ( v = 1,2 J = 2-1) SiO masers. All the transitions have been imaged except the v = 2 J = 2-1 in IRC + 10011. We report the first VLBI map of the v = 1 J = 2-1 28SiO maser in IRC + 10011 as well as the first VLBA images of SiO masers in an S-type Mira variable, cCyg. In this paper we have focused on the study of the relative spatial distribution of the different obsd. lines. We have found that in some cases the observational results are not reproduced by the current theor. pumping models, either radiative or collisional. In particular, for IRC + 10011, the v = 1 J = 1-0 and J = 2-1 28SiO lines have different spatial distributions and emitting region sizes, the J = 2-1 emission being located in an outer region of the envelope. For cCyg, the distributions also differ, but the sizes of the masing regions are comparable. We suggest that the line overlaps between ro-vibrational transitions of two abundant mol. species, H2O and 28SiO, is a possible explanation for the discrepancies found between the observations and the theor. predictions. We have introduced this overlapping process in the calcns. of the excitation of the SiO mol. We conclude that the line overlaps can strongly affect the excitation of SiO and may reproduce the unexpected observational results for the two sources studied. [on SciFinder (R)]